Tall or short? A pea plant cross

In this video, we use a Punnett square to find the fraction of short offspring.

Question one. In pea plants, tall, represented by capital T, is dominant over short, represented by lowercase t. Cross capital T lowercase t with capital T lowercase t: what fraction of the offspring is short?

Both parents are heterozygous, with genotype capital T lowercase t. We need the fraction with the short phenotype. Because tall is dominant, only lowercase t lowercase t offspring are short.

Each parent produces capital T and lowercase t gametes in equal proportions. Place one parent's gametes across the columns and the other's down the rows. The first row gives capital T capital T and capital T lowercase t.

The second row gives capital T lowercase t and lowercase t lowercase t. Only one of the four equally likely combinations is lowercase t lowercase t. So the fraction of short offspring is one quarter, or twenty-five percent.

The other three combinations contain a dominant allele and produce tall plants. Hence, the answer is one quarter of the offspring.